This invention relates to a method of desensitizing carbonless paper and, more particularly, to preventing image formation in selected areas of carbonless paper. In a typical carbonless paper form, the back side of the top sheet has a coating of microscopic capsules containing colorless chromogens or color precursors in the oil. This top sheet is called CB paper, for coated back. The bottom sheet is the receiving sheet which is coated with a color developer. It is designated as CF paper, for coated front. The intermediate plies have the color developer coating on the front side and microcapsules coating on the back side. These sheets are called CFB. A self-contained paper has both the color developer and microcapsules coated on the same side of the web. The color precursor is an electron donator while the color developer is an electron acceptor.
There are three kinds of color developers commonly used in the receiving sheet of carbonless paper system: the acid clays, such as attapulgite clay, Silton clay, acid leached kaolin clay, and the like; phenolic novolac resins, such as zinc modified novolac of p-phenylphenol, p-octylphenol, p-tert-butylphenol, and the like; metal salts of organic carboxylic acids, such as zinc salts of 3,5-di-tert-butylsalicylic acid, 5-octylsalicylic acid, 4,7-diisopropyl-1-hydroxy-2-naphthoic acid, and the like.
The chromogens which react with the color developers to form images include phthalides, such as 3,3-bis (p-dimethylaminophenyl)-6-dimethylaminophthalide, 3,3-bis (p-dimethylaminophenyl) phthalide, and 3,3-bis (1-ethyl-2-methylindol-3-yl) phthalide: derivaties of Michler's hydrol, such as benzyl ether of Michler's hydrol, p-toluene sulfinate of Michler's hydrol, and morpholine derivative of Michler's hydrol; fluorans, such as 6-diethylamino-2-chloro-3-methylfluoran, 6-diethylamino-2,3-dimethylfluoran, 6-diethylamino-2-methylfluoran, 6-diethylamino-2-dibenzylaminofluoran, 6-diethylamino-2-anilino-3-metylfluoran, 2,2'-bis (6-diethylaminofluran), and 6-diethylamino-2-acetylmethylaminofluoran; and lactams, such as rhodamine-B-anilinolactam, rhodamine-B-(p-nitroanilino) lactam, and rhodamine-B-(p-chloroanilino)-lactam.
There are water base and ink base desensitizers known in the prior art. Water base compounds are much less effective for the organic acid developers such as phenolic novolac and metal salts of salicylic acid derivatives. The ink base desensitizers remain tacky for a long period of time after being applied to the web and this is particularly undesirable when the web roll is running at production speed. The ink base material tends to adhere to the back side of the web, thereby being inadequate for wholly covering a selected portion of the CF sheet. Then, when image production occurs, viz., typing or printing, unwanted images appear because a portion of the desensitizer has been carried away on the back of the web.
The purpose of this invention is to desensitize certain desired areas containing the color developer (whether CF or selfcontained) so that the image development is prevented. The desensitizers are formed in situ via free-radical induced polymerization. More particularly, the desensitizer composition is the copolymer of N-vinylpyrrolidone and ethylenically unsaturated oligomers of the following formula: ##STR1## wherein R.sup.1 is a hydrogen atom, or an alkyl group having from 1 to 4 atoms; R.sup.2 represents a block of the ethylenically unsaturated compound, such as acrylates and vinyl compounds; and n is an integer of 1 to 10. Thus, the free-radical co-polymerizable compounds have at least one terminal ethylenic group per molecule.
As indicated above, the co-polymerizable compounds are acrylates and olefinic compounds. The preferred acrylates are monofunctional acrylates, such as methyl acrylate, methyl methacrylate, lauryl acrylate, 2-ethylhexyl acrylate; polyfunctional acrylates, such as ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, 1,6-hexanediol diacrylate, trimethylol propane trimethacrylate, trimethylol propane triacrylate, pentaerythritol triacrylate, neopentyl glycol diacrylate, and the like. Mixture of these acrylates and their oligomers may also be used. The oligomers include acrylated resin RR 27441 (Celanese Chemical Company, New York), acrylated resin RR 27418 (Celanese Chemical Company), Celrad 3699 acrylated resin (Celanese Chemical Company, CK-7623 clear UV varnish (Inmont Corporation), Acrylated Resin DRH 370 (Shell Chemical Company), Acrylated Resin SA 9928 (Sherwin-Williams Company), and Celrad Acrylated Resin 3700 (Celanese Chemical Company). The preferred olefinic compounds are acrylonitrile, isobutyl vinyl ether, vinyl acetate, vinyl laurate, vinyl stearate, styrene, biallyl, diallyl phthalate, divinyl adipate, divinyl benzene, diethylene glycol bis (allyl carbonate), butane divinyl ether, and the like.
A photoinitiator is employed to induce the polymerization upon the exposure to radiation such as ultraviolet light or electron beam. The preferred photoinitiators are chlorothioxanthone (Ultra-Cure I-100 manufactured by Sherwin Williams Company), benzoin butyl ether (Vicure 19, Stauffer Chemical Company), a mixture of alkylbenzoin ethers (Vicure 50, Stauffer Chemical Company), benzophenone, 4,4'-bis (dimethylamino) benzophenone, and azobisisobutyronitrile.
Additionally, the photoinitiation may be enhanced by the addition of synergists, such as dimethylaminoethanol, diethylaminoethanol, N-methyl diethanolamine, and N-methylmorpholine.